Uses of polyamides

The properties of the polyamides (nylons), which include high strength, abrasion resistance, and resilience, make them very important in the manufacture of clothing and carpets. Although these polyamides account for 95% of the material used in women's hosiery, this still only accounts for about 5% of the total fibres used to make clothing. Nevertheless this is more than either the polypropenoates (acrylics) or wool but it is substantially less than either cotton or polyesters.

Figure 1 The children's clothing is made of polyamide 6, impregnated with nanoparticles of titanium dioxide which gives protection against UV radiation, a very effective way of having a sunscreen.By kind permission of BASF.

The polyamides (nylons) are also used in engineering plastics, for example, in cars, and for making films for food packaging. They are used in films for their good balance between mechanical strength and barrier properties against oxygen, smells and oils.

Figure 2 An important development is the use of polyamides to make safety airbags.By kind permission of the Delphi Automotive.

Polyamide 6,6 was first produced in the laboratory in 1935 by W H Carothers whilst working for DuPont in the US. Commercial production started in 1938, the same year as I G Farbenindustrie developed polyamide 6 in Germany.Polyamides 6 and 6,6 are the most widely used polyamides for fibres and for engineering materials. The remaining commercial polyamides, for example, polyamides 11 and 12, and 6,10, are most used as engineering plastics.

Figure 3 Ropes made from polyamides are used by rock and ice climbers. They are not only very strong but they are also stretchy and thus reduce forces in the event of a fall, by spreading the duration of loading transmitted to anchors and to the body via the harness. Tony Moody is climbing ice on Heninger, near Cogne in northern Italy.By kind permission of Tony Moody.

For use as an engineering plastic, polyamides are often compounded with fillers, pigments, glass fibre and toughening agents to give specific properties to the polymer. However, for either continuous filament or staple fibres, which are melt spun at very high speeds (ca 6 km every minute), there is great emphasis on controlling the polymer chemistry and the way the yarn is produced in order to ensure the production of the high quality material needed for particular purposes. For example, the thread for use in stockings needs to be strong, as well as very fine, so the molecular mass and hence tensile properties of the polymer must be carefully controlled.

Annual production of polyamides

Polyamide 6,6

World

3.4 million tonnes

Europe

700 000 tonnes

Polyamide 6

World

4.3 million tonnes

Europe

1.2 million tonnes

Caprolactam (monomer of polyamide 6)

Europe

1.1 million tonnes

US

800 million tonnes

FSU

500 million tonnes

China

460 million tonnes

Rest of Asia

1.2 million tonnes

Manufacture of polyamide 6 and 6,6

Both polyamides are manufactured from benzene via cyclohexane. Hydrogen is passed through liquid benzene in the presence of a nickel catalyst under pressure:

Cyclohexane is oxidized by passing air through the liquid under pressure in the presence of a catalyst (often a cobalt salt) to yield two products:

The mixture of cyclohexanol and cyclohexanone is known as "mixed oil" or KA (ketone/alcohol).An alternative route to cyclohexanol is via the hydrogenation of phenol using a nickel catalyst at ca 400 K and 5 atm:

A more recent route to cyclohexanol is the Asahi process from benzene via its hydrogenation to cyclohexene and subsequent hydration to alcohol. This is more energy efficient than the other processes.

To make polyamide 6, pure cyclohexanone is required. When the mixed oil is heated under pressure with copper(ll) and chromium(lll) oxides, the cyclohexanol, which is a secondary alcohol, is dehydrogenated to the corresponding ketone, cyclohexanone:

Cyclohexanone is then converted into caprolactam via the oxime (produced by the reaction of the ketone with hydroxylamine - in the form of the salt, hydroxylamine hydrogensulfate):

The isomerisation of the oxime to caprolactam by sulfuric acid is an example of the Beckmann rearrangement in which an oxime is transformed into an amide in the presence of acid.A zeolite, with acidic sites, is also being used to effect the rearrangement. The zeolite is regenerated and saves the use of sulfuric acid.To produce the polymer, the caprolactam, water (acting as a catalyst) and a molecular mass regulator, e.g. ethanoic acid, are poured into a reaction vessel and heated under nitrogen at 500 K for about 12 hours:

Polyamide 6,6 is produced by reacting 1,6-diaminohexane (hexamethylenediamine) with hexanedioic acid (adipic acid) by condensation polymerization.

One of the monomers, hexanedioic acid is also produced from KA mixed oil (cyclohexanol and cyclohexanone). The mixed oil is oxidized in the liquid phase using moderately concentrated (60%) nitric acid and a copper(II) nitrate and ammonium vanadate(V) catalyst, at 330 K to form hexanedioic acid:

This process has a considerable disadvantage. A side-product is nitrogen(I) oxide (nitrous oxide), N2O, a powerful greenhouse gas but it is carefully removed by thermal or catalytic treatment units.The second monomer, 1,6-diaminohexane, is produced from buta-1,3-diene and from propenonitrile (polyacrylonitrile).To form the polymer, the acid and the diamine are then heated together to form a salt.The chemical reaction for aliphatic dicarboxylic acids and aliphatic diamines to yield an aliphatic polyamide via a condensation polymerization process can be represented, thus:

The chain length is regulated by controlling process conditions, such as reaction time, temperature and pressure. An aqueous solution of the salt is heated, in the absence of air, to ca 500 K. A pressure develops in the vessel. The temperature is then raised to 540 K, and the steam is bled off to keep the pressure constant. Eventually, the pressure is reduced and the polymer is extruded under nitrogen to yield a lace which is then granulated (Figure 4).

Other polyamides

Figure 4 The granules are a polyamide from which the frames of the glasses have been moulded.By kind permission of Arkema.

Date last amended: 7th May 2013

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